UV's Changing from square to rectangular texture
The texture is mapped to the UV coordinates so that the lower left corner is on 0,0 and the upper right is at 1,1.

So if you extend your image you will need to scale the UV coordinates to match the new mapping coordinates. The scaling origin depends on how you have extended the texture. Place the cursor in the uv editor based of that and use the cursor as pivot for the scaling.

You can calculate the scaling by dividing the old texture size with the new.